I saw someone in the Fighter thread saying that Fighter was the class that 4e did best. I think there's a lot of validity to that - while caster implementation was divisive for people, the Martial classes from the 4e PHB all did a great job of making their classes step up and be more valuable, awesome, and plain fun to play than in previous editions. Essentials proved that a lot of that power and fun could be retained while also creating classes that were on the face of them simpler to operate (though still with depth of tactical play).
However, the true breakout success class of 4e for me was the Warlord. While they may have made for some flavor humor occasionally ("**** it Elgin, rub some dirt on it and get back up!" was my frequent "healing word" to our unconscious fighter), breaking out the ability to heal and buff the party from just the cleric was a huge step forward.
The overall concept of "not needing a cleric" is something the designers are clearly thinking about it (
Dungeons & Dragons Roleplaying Game Official Home Page - Article (Rule-of-Three: 01/10/12) ), but in addition, I think the Warlord class itself, as the flag bearer of the evolution from "healer" to "Leader", with their diverse buffs, heals, and ability to fight on the front line beside the warriors, is worthy to stay in the "Core" set of classes. (Warlocks, however, can safely vanish to some remote splatbook.) Who's with me?
(This assumes that we have more than 4 core classes, of course - if we only get 4, stay with the originals - cleric, halfling, elf, dwarf . . . I mean cleric, thief, mafe, fighter)